A prior art amplifier circuit including an operational amplifier has significant disadvantages caused by the bias currents of the operational amplifier, which are not negligible for circuit operation.
In a noninverting amplifier circuit composed of an operational amplifier, for example, input signals are applied to the noninverting input port of the amplifier through a resistance. The bias currents flowing from or to the noninverting input cause a voltage across the input resistance. The operational amplifier produces its output as if the voltage due to the bias currents were applied to the noninverting input port. If the input signal source has a high output impedance, accurate amplification of the input signals will not be expected.
With a conventional integration circuit using an operational amplifier, the bias currents of the amplifier make the voltage across a capacitor drift, which interconnects the output port with the inverting input port of the operational amplifier. Thus, the drift in capacitance voltage prevents the output integrated voltage of the amplifier from being kept constant even if the inverting input terminal is maintained open.
In a prior art amplifier circuit of high input impedance, the noninverting and inverting input terminals of the operational amplifier are respectively connected to the intermediate node of the respective voltage dividers, which are commonly coupled with the output port of the amplifier and the ground. It is well known to those skilled in the art that, in order to compensate for the bias currents of the amplifier, each of the resistors which respectively interconnect the noninverting and inverting input terminals to the ground are selected in such a fashion that the ratio of those resistances are equal to that of the associated bias currents of the amplifier. However, it is very difficult to determine the ratio of those bias currents since it is difficult to measure the bias currents which are on the order of several ten or several hundred nanoamperes.